Usually the authors of a plugin provide the correct folder structure already but here is a very simple explanation of the folder structure.
Code:
csgo //main mod directory of the game css & 1.6 = cstrike, csgo = csgo, team fortress 1&2 = tf etc.
-addons //main addon directory, meta and sourcemod go in here
--metamod //metamod relevant files
--sourcemod //sourcemod relevant files
---configs //sourcemod and plugin config files
---data //client preferences and some other plugin data is stored in here
---extensions //sourcemod extension files go in here (linux files ending with ext.so and for windows ext.dll)
---gamedata //game data files (.games.txt) go in here
---logs //sourcemod log file very helpful if a plugin doesn't work
---plugins //the actual sourcemod plugin files (.smx) go in here
---scripting //sourcemod source code files of plugins are stored in here (these files are only needed for compiling a plugin and don't have to be upload to the game server for the plugin to work)
---translations //sourcemod translation files (.phrases.txt) go in here
-cfg //most config files are located in here
--sourcemod //anohter location of sourcemod and plugin config files
Basically all you have to do is matching the folders.
If your plugin source doesn't already provide you the right folder structure you can use the explanation above to figure out where to move the files.
Code:
folders
configs > csgo/addons/sourcemod/configs
gamedata > csgo/addons/sourcemod/gamedata
translations > csgo/addons/sourcemod/translations
single plugin file
X.smx > csgo/addons/sourcemod/plugins